The prior art has known the use of a rotating flat mirror at the focal point of a parabolic mirror to scan a hot glowing object with the scan falling on a photocell. The time from scan of one side to the other of the object indicates its dimension.
Another prior art apparatus is one wherein a rotating mirror is provided at the focal point of a parabolic reflector. Part of the beam is reflected from a raster reflector and the larger part of the beam passes through the field of scan of an object and is then reflected from retroreflective material. Pulses are counted to determine the dimension of the object.
Another prior art apparatus utilizes a rotating mirror and lens to scan an image of an object onto a code plate having a striped pattern, the scan of which converts the covered or uncovered stripes into a corresponding number of pulses which are then counted to be an indication of the dimension of the object.
Another prior art apparatus utilizes a laser and a rotating mirror to a beam splitter and lenses to establish parallel laser beams to scan an object in two mutually perpendicular directions. A master clock creates pulses as the object is scanned while the photocell is dark, the time being determined by the number of pulses and this gives the diameter as the workpieces scan from one side to the other.
Another prior art apparatus scans hot bars, with the light beam from the two sides of the hot bar passing through a lens and then onto two separate diode arrays. The shaded parts of each array are then counted for two complementary portions of the dimension of the bar.
The difficulty with the prior art systems is that most are not usable in a hot, dirty or otherwise hostile environment because parts of the system must be located close to the workpiece, or parts of the system must be on opposite sides of the workpiece, or the dimension of the workpiece must be small to be smaller than a lens used in the system, or the system depends upon infrared radiation from the object hence it must be hot and glowing, or the system will not operate with sufficient accuracy, or will not measure a vibrating or rapidly moving object, or they scan from one side of the object to the other without any reference beam.